Magnetic stirring apparatus



Aug. 15, 1950 G. B. COOK MAGNETIC STIRRING APPARATUS Filed June 22, 1949 ZSnventor :9 e B. 00 04%,

Geo Bg/0 Www attorney Patented Aug. 15, 1950 UNITED STATES PATENT OFFICE 21,518,758 MAGNETIC STIRBIN G APPARATUS George 3. Cook, Philadelphia, Pa. Application June 22, 1949, Serial No. 100,567

11 Claims.

The object of the invention is to provide improvements in stirring devices, and while the invention has a wide range of use, it has been designed and perfected particularly with respect to and is eminently adapted for use in the stirring of liquids, whether highly fluid or relatively sluggish, in laboratory practice.

It has been well known for several years that a magnetized or magnetizable bar or the like, when placed in an ordinary glass flask within a revolving magnetic field, will tend to revolve in substantial synchronism with said field, and at the same time stir whatever liquid substance may also be within said flask. A magnetically actuated bar must necessarily be of metallic nature, that is, made of iron, steel, or a, magnetizable alloy, etc., but due to the fact that strong acids or alkalis surrounding the bar within the flask attack a metallic bar and produce contamination of the fluid, such a bar must obviously be covered by glass or certain plastics, though a glass covering is adapted for the widest range of use by reason of the fact that plastics cannot be counted upon to the same degree to withstand chemical reaction with the flask's contents. But in any case, direct contact of the metallic bar or its protective covering with the glass surface of the flask under the influence of the magnetic field, endeavors to draw the bar towards the magnetic center of such field, and produces such friction that movement of the bar is slowed and both the flask and the covering of the bar become scratched to a dangerous degree until one or both become damaged beyond repair.

Another and more specific object therefore is to provide a shape 01' envelope for the bar, that while it is completely protected from reaction with whatever contents may be in the flask, and regardless of whatever ordinary cross section may characterize the bottom of the flask, friction between the bar envelope and the flask will be lessened to a practically irreducible minimum, and the rotation of the bar and its enclosure permitted to respond to and in substantial synchronism with the motion of the adjacent or surrounding field, at the same time reducing to nil the wear of the bar envelope upon the flask, since from the characteristic shape of the improved envelope 1;, portion of the liquid being stirred is constantly between the envelope and adjacent flask surface, such portion decreasing in thickness radially inwardly until it approaches zero at an area that is theoretically of an infinitesimal diameter.

A further and more specific object is to provide in combination with an elongated magnetized or magnetizable bar a protective envelope, that is formed 01' the well known Pyrex or similar type of hard glass, having the general shape of an ellipsoid but with preferably more pointed extremities and a more abruptly curved central portion than characterizes a true ellipsoid, the more pointed extremities eilfecting a better stirring action while the more abruptly curved central portion provides a greatly narrowed line of tangency about the smaller axis of said envelope, any one point of which line can serve as the pivotal point of minimum contact about which said stirrer unit spins upon the bottom of the flask, it being noted that the curvature of the flasks bottom wall has a radius that in every case is substantially greater that that of said envelope, or may even be flat except that an actually flat bottom in the flask would not aid in centralizing the axis of rotation of the stirrer coaxial with that of the flask and the rotating magnetic fleld. It has been found that, as a result of thus minimizing the friction between envelope and flask, the flask may be raised higher than is customarily possible, as for example when a glass basket heating unit or other intermediate supporting element is positioned between them.

With the Objects thus briefly stated, the invention comprises further details of construction and operation, such as are hereinafter fully brought out in the following description, when read in conjunction with the accompanying drawings, in which Fig. 1 is a side elevation of a stirring device comprising one embodiment of the invention, the flask, supporting basket and motor-magnet housing being broken away to show the interiors thereof; Fig. 2 is an enlarged fragmentary portion of the lower part of the flask shown in section with one form of stirrer element therein and having its envelope broken away to show the bar magnet within; Fig. 3 is a vertical section on the line 3--3 of Fig. 2; Fig. 4 is an elevational view of a modified form of stirrer element per se, with its envelope partially broken away to show the metallic particles within; and Fig. 5 is 'a central longitudinal section of a modifled form of said element in which the ends of the bar magnet are slightly spaced from the adja-- cent inner surface of the surrounding envelope, and maintained in such position by means of suitable cushioning material.

Referring to the drawings, a bar or similar shape of magnet l is mounted upon a vertical shaft 2, that is driven by a motor 3. The magnet may comprise a permanent magnet of steel in an inverted cup-like non-magnetic casing l that is secured by screws or the like to a base 6. From the motor extends any suitable form of double conductor 1 having a terminal plug 8 for connection with a suitable source of current, and preferably also with a resistance unit 9 for varying the speed oi! the motor at will and thereby the speed of rotation of the magnet within the flask.

Upon the top of said casing and serving as a plaform may be placed a semispherical basket ill of spun glass or asbestos, within which is embedded an electric heater unit II, that for convenience may be carried by and within the weave of a coarse textile fabric made of spun glass threads, asbestos, or other suitable non-inflammable substance. From such heater unit extends a flexible electric cord I2 with terminal contacts [3 for connecting said unit to a suitable source of current. Such a unit may be provided also with an automatic thermostatic control for maintaining a constant temperature.

Within and substantially half surrounded by said basket is the lower substantially spherical body portion of a common laboratory flask H, from which the usual reduced filling neck I5 extends, either vertically or diagonally upwardly. Within said flask may be placed any liquid that it is desired to heat and maintain heated by said resistance unit, or permitted to remain at room temperature if preferred, while lying in the bottom of said flask is a stirrer element that comprises the essence of the invention.

The stirrer element shown in Figs. 1, 2 and 3 comprises a substantially ellipsoidal envelope l6 of-such heat and chemical resistant material as Pyrex glass, or some suitable form of plastic where such material will not be affected by or contaminate the contents of the flask, within which is preferably axially positioned a bar magnet l'l. While a bar-shaped magnet has been used heretofore in testing the invention, the magnet if preferred may take the shape of the interior of said envelope, in which case its lowermost portion will more closely approach the plane of rotation of the motor-driven magnet I, and thereby be more positively influenced by the rotating magnetic field thereof.

Referring to Fig. 4, there is here shown a substantially ellipsoidal envelope l8, which is substantially filled with steel filings or finely subdivided particles IQ of Alnico or other highly magnetizable metal or metallic alloy. Such a stirrer element may be prepared by first blowing or otherwise forming the envelope with a small opening preferably in one ,end thereof, through which the filings or other form of small particles are inserted, after which the hole is closed by fusing or is otherwise sealed. In any case, the body of metal, whether a solid bar or a body of small subdivided particles, may be remagnetized by placing it in a strong unidirectional, non-fluctuating field.

The term ellipsoid, or ellipsoidal, as used herein relates to a stirring element having an'elongated major axis and all of the plane sections of which parallel with said major axis are substantially ellipses, while all of the plane sections.

at right angles or perpendicular to sa d m jor axis are substantially circles. Thus, the stirring element of the invention-is not limited to this exact geometrical figure, but is represented by the same, while irrespective of the exact shape of the opposite poles of said element, the intermediate portion is preferably substantially circular in planes perpendicular to said major axis, and iongitudinally is of a curvature that is greater than that of the inner surface of the bottom of a flask or other vessel in which it is intended to function, so that the locality of contact between said element and said vessel is as nearly that of a point as possible, in order to thereby reduce friction to the lowest possible minimum. Thus, the equator of said'element theoretically consists of an infinite number of consecutive points, any one of which and one alone may serve as the pivotal support for said element, as it revolves upon the floor or bottom of a curved vessel about an axis that is perpendicular to said major axis.

References herein to magnetizable metals is intended to include iron, steel and various alloys of which an aluminum-nickel-cobalt alloy marketed under the trade name Alnico is probably the best known andthe most densely or strongly magnetizable. Heretofore a solid bar magnet has been used, and in order to bring its center of mass as low as possible into the magnetic field of the driving magnet beneath it, such a bar has been bisected longitudinally. However, even a bisected bar with its flat face directed downwardly is still higher than is desirable, due to the engagement of its ends with the curved sides of the vessel. Therefore, it has long been realized that any means whereby the center of the magnetized mass could be lowered farther into the rotating magnetic field beneath it would be a distinct advantage.

Such a mass can be obtained by preparing a hard glass capsule of the desired shape, preferably of Pyrex or similar glass and having a small filling hole initially left in one end, filling said capsule with finely subdivided particles of the proper metal or alloy, sealing said filling aperture and then magnetizing the metallic mass in the field of a strong electro-magnet or equivalent source of magnetism. In lieu of filings small balls or the like may be used.

The material of the envelope surrounding the magnetizable bar or other mass of metal is preferably of Pyrex glass, and probably in all cases would be a glass of some sort, as glass is probably the only practical medium for enclosing the stirrer element, since it is impervious to attack by practically all chemicals except one or two exceptionally strong acids. However, as glazed porcelain might be used the term vitreous, is herein used to include glass, porcelain, and other possible substances that are impervious to chemicals generally.

In the operation of the stirrer device as a whole. the liquid to be stirred is placed within the flask M, which may be supported by an adjustable bracket such as is a part of the equipment of all laboratories, instead of by the basket l0 as illustrated. In the first instance the flask and its contents may be heated in well known manner by a Bunsen burner, whereas in the latter case they are heated by the resistance unit within the body of said basket. With the stirrer element lying upon the bottom of the flask, the motor is rotated at the desired speed under control of the rheostat 9, and the magnet l similarly rotated,

thereby causing the stirrer element to rotate ll l th in uence 0f the revolving magnetic field set up by and in the vicinity of the magnet I.

The driving magnet I may be of any desired shape, but in any case must have two oppositely directed magnetic poles, such as are probably best defined by the so-called north and south polar ends of a bar magnet. The envelope l8 instead of being of substantially ellipsoidal or football shape may have any desired shape, as long as it is characterized by two oppositely directed magnetic poles and is so curved.as to provide a minimum area of contact for pivotpoint engagement with the bottom of the flask upon which it rests and normally rotates.

Also, in the case of a bar magnet being enclosed within the improved envelope, it has been found advisable to surround at least the end portions of such magnet with glass wool 20 (Fig. for the purpose of cushioning it in the event of sudden shock, as for example when the element is dropped upon inserting it in a flask, or when accidentally dropped upon a desk, table, or floor. In this way the glass of the envelope is much less likely to fracture, and in any case the relatively sharp edges of the magnet will not be so likely to score the inner surface of said envelope.

Having thus described my invention, what I claim and desire to protect by Letters Patent of the United States is:

1. The combination of a vessel having a nonconvex inner bottom surface and a revolving magnetic field adjacent thereto, with a bar of magnetized metal within said vessel, and a generally ellipsoidal enclosure for said bar, the curvature of the external surface of which at its equator in the direction of its longitudinal axis is defined by a radius less than that of the inner surface of said vessel.

2. The combination of a vessel having a concave inner bottom surface and a revolving magnetic field adjacent thereto, with a bar of magnetized metal within said vessel, and a non-magnetic generally ellipsoidal enclosure for said bar, the curvature of said enclosure from end to end being greater than that of the inner surface of said vessel.

3. A device for stirring the liquid contents of a vessel about a generally vertical axis, comprising a magnetized bar within said vessel, and a nonmagnetizable casing for said bar that is substantially circular at its equator and in contact with only the ends of said bar, and whose curvature longitudinally is of greater degree than that of the inner bottom surface of said vessel, and a rotating magnetic field extraneous to said vessel causing said bar and said casing to revolve therein.

4. A device for stirring the liquid contents of a vessel about a generally vertical axis, comprising a magnetized bar within said vessel, and a vitreous casing for said bar that is substantially circular at its equator and in contact with only the ends of said bar, and whose external curvature is such that it is adapted to rest upon and with an infinitesimal point of contact with the field and a non-magnetic vessel in inner bottom surface of a supporting vessel, and a rotating magnetic field extraneous to said vessel causing said bar and said casing to revolve therein.

5. A device for stirring the liquid contents of a vessel, comprising a non-magnetic envelope of substantially ellipsoidal shape, and small particles of a magnetized substance substantially filling said envelope.

6. A device for stirring the liquid contents of a vessel, comprising a non-magnetic envelope, and small particles of a magnetized substance substantially filling said envelope.

7. A device for stirring the liquid contents of a vessel about a generally vertical axis, comprising a non-magnetic envelope of substantially ellipsoidal shape, and a mass of a magnetized substance within said envelope, the lcngitudinal curvature of said envelope being greater than that of the inner surface of a flask upon which it is intended to rest.

8. The combination of a rotating magnetic field and a non-magnetizable vessel in said field, with a permanently magnetized mass within said vessel, and a substantially ellipsoidal enclosure surrounding said magnet, that rests upon and has a one-point contact with said vessel.

9. The combination of a rotating magnetic field and a non-magnetizable vessel in said field, with a permanent bar magnet within said vessel, and a substantially ellipsoidal enclosure surrounding said magnet, that rests upon and has a one-point contact with said vessel.

10. The combination of a rotating magnetic said field, with a permanent bar magnet within said vessel, and a substantially ellipsoidal enclosure surrounding said magnet, that rests upon and has a one-point contact with said vessel, said enclosure engaging only the end portions of said magnet.

11. The combination of a rotating magnetic field and a non-magnetizable vessel in said field, with a permanent bar magnet within said vessel, and a substantially ellipsoidal enclosure surrounding said magnet, that rests upon and has a one-point contact with said vessel, said enclosure being peripherally spaced from the central por-- tion of said magnet, and a shock absorbing medium between said envelope and the ends of said magnet.

GEORGE B. COOK.

REFERENCES CITED The following references are of record in the file of. this patent:

UNITED STATES PATENTS Number Name Date 383,020 Sherman May 15, 1888 1,607,828 Holthofi Nov; 23, 1926 2,466,468 Neal Apr. 5, 1949 FOREIGN PATENTS Number Country Date 424,564 Germany Jan. 26, 1926 

